IF and WHEN you decide to out the affair, I highly recommend just putting a book on you bed "Surviving Infidelity" next to her early in the morning, and leave for work early. Let her see the book and think about how she's gonna respond without you even being there. Cant start a fight that way. She'll have the whole day to think about it before you get home from work. That may or may not be soon, but its a thought about how.
I'm sorry your here, Hopefully you can futher muster the strength to see your way thru it.